This invention is in the field of fuel cell control systems and more particularly to a method and system for controlling a voltage output of a fuel cell by varying a number of input parameters.
Fuels cells operate by generating electricity electromechanically. A fuel and an oxidant are provided to the fuel cell where they react in the presence of an electrolyte to generate electricity. Although various fuels and oxidants can be used, the most common are likely hydrogen and oxygen because the byproducts of a hydrogen fuel cell are rather harmless; mostly just water.
In a typical hydrogen/oxygen fuel cell, the hydrogen and oxygen are initially separated in the fuel cell by a membrane. The hydrogen, reacting with a catalyst, disassociates into protons and electrons. The disassociated protons pass through the membrane to the oxygen on the other side of the membrane, while the electrons are used to power an external circuit. On the oxygen side of the membrane, the protons, oxygen and electrons (which are reintroduced after traveling through the external circuit) react to form water and water vapor before the water and water vapor exits the fuel cell.
Current fuel cell designs are rather static; that is, they are designed to deliver power output which is preset to a certain value or within a relatively narrow power range. These fuel cell designs that deliver a fixed power output normally operate at a set temperature with a constant membrane thickness and fuel concentration. The fuel cell designs that allow a limited capability to deliver power in a relatively narrow range normally operate at a fixed temperature keeping constant membrane dimensions, such at thickness, and varying the fuel concentration. However, these designs do not have a high degree of flexibility in their ability to vary the fuel concentration because too much variation will cause fuel crossover through the membrane, which will decrease the fuel efficiency and adversely affect the performance of the fuel cell. Therefore, the variation of fuel concentration of these fuel cells is somewhat limited which in turn limits the operating range of the fuel cell.
Typically, for applications where substantial variations in voltage/current output is required, complex electrical systems are required after the fuel cell which requires power conversion from the output of the fuel cell to be compatible with the load being driven. These system usually also include a battery system to provide short bursts of increased demand. The fuel cell design and inputs am typically designed in these systems to produce a voltage output that is usually greater than the demanded load, so that system can step down the power output from the fuel cell to provide the necessary voltage demanded. However, in this method, some efficiency is sacrificed because the fuel cell is purposely kept in an overproducing state.
There have been some attempts to increase the input parameters of fuel cells, for example, U.S. Pat. No. 6,794,855 to Hochgraf et al. discloses a control system for a fuel cell that provides temperature control in addition to controlling fuel and oxygen input, but is still limited in the input parameters it can solve for and control in real time.